This invention relates to an improved process for purifying thionyl chloride by the double distillation of an impure mixture of thionyl chloride in the presence of sulfur. The thionyl chloride produced by this process is extremely pure and therefore suitable for uses requiring low levels of impurities.
Thionyl chloride (SOCl.sub.2) is a chemical compound which is widely used in the preparation of chemical intermediates for pharmaceuticals, dyestuffs and related chemicals. It is a versatile chlorinating agent which reacts with a variety of organic functional groups, and often is the reagent of choice for this purpose since the by-products are gases which are easily removed from the reaction product. A highly pure product is important to many customers since small amounts of impurities will cause an objectionable discoloration of the product.
Thionyl chloride is prepared commercially by a number of different processes. One process involves the vapor phase reaction of sulfur monochloride (S.sub.2 Cl.sub.2), sulfur dioxide and chlorine over an activated carbon catalyst. This reaction is shown below: ##STR1##
The impurities found in products produced according to reaction (1) are mainly mixtures of sulfur chlorides, e.g. sulfur monochloride and sulfur dichloride, sulfuryl chloride, sulfur dioxide and chlorine. The atmospheric boiling points of these components are as follows:
Thionyl chloride: 78.degree. C. PA1 Sulfuryl chloride: 69.degree. C. PA1 Sulfur monochloride: 138.degree. C. PA1 Sulfur dichloride: 59.degree. C. PA1 Sulfur dioxide: -10.degree. C. PA1 Chlorine: -35.degree. C.
The chlorine, sulfur dioxide and sulfur monochloride components can be separated from the mixture with relative ease due to the difference in boiling points. However, the sulfur dichloride and sulfuryl chloride are not that easily separated, particularly when a high purity product is required. This problem is compounded by the fact that at normal distillation temperatures thionyl chloride decomposes in the presence of sulfur.
Complete purification of the product by simple distillation is very tedious, requiring many steps and the removal of by-products. Decomposition of the thionyl chloride may occur under these conditions, further hampering purification and reducing the yield. Multiple distillations offer incremental improvements in product purity but do not result in significant reduction in the sulfuryl chloride content. These approaches are therefore not considered satisfactory from a commercial standpoint.
Methods of purifying impure or crude thionyl chloride have been developed using organic compounds such as styrene, octene, and decene. Although these compounds react with the sulfur chlorides to remove some impurities, they also result in a significant decrease in thionyl chloride yield, leave an unstable residue in the product, and increase its organic carbon content.
The addition of sulfur and aluminum trichloride to the distillation column is disclosed in U.S. Pat. No. 4,337,235 to Wisnouskas, issued June 29, 1982, and U.S. Pat. No. 4,388,290 to Jonas, issued June 14, 1983. This process results in the reduction of both sulfur chlorides and sulfuryl chloride impurities. However, this approach is too costly for commercial use.
A procedure commonly used today is the double distillation of impure thionyl chloride in the presence of sulfur. This technology is described in U.S. Pat. No. 3,155,457 to Kunkel, issued Nov. 3, 1964, the disclosure of which is incorporated herein by reference. This process is effective in converting sulfur dichloride to sulfur monochloride which can then be readily separated by distillation, but it does not remove significant amounts of sulfuryl chloride.
None of the above described processes are entirely effective in purifying crude thionyl chloride either due to ineffectiveness or poor economics.
It is therefore a principle object of the present invention to provide a cost effective thionyl chloride purification process which will produce a thionyl chloride product which is suitable for use in applications requiring very low levels of impurities.